1. Field of the Invention
The present invention relates to a system and method for starting rotation of a disc in a disc storage unit such as a hard disc unit, and more particularly to a system and method for starting rotation of a disc smoothly when it is difficult to be driven due to adsorption or adhesion of a head.
2. Description of the Prior Art
Disc storage units have been widely used not only in so-called office computers but also recently in personal computers and word processors. At present magnetic recording type disc storage units are mainly used although it is expected that attention will be paid to optomagnetic recording types in the future. As is well known to one skilled in the art, in a magnetic disc storage unit, data information is read out and written from and into a magnetic recording medium on a disc via a head, which is a kind of a magnetic transducer. In its active state, the head either contacts a surface of the disc as is the case of a floppy disc or is slightly lifted from a surface of the disc by air pressure as is the case with a hard disc.
Even in the latter case, however, the air pressure for lifting the head is generated by rotation of the disc and therefore the head is in contact with the surface of the disc in a stop mode when the disc is stationary, and the head sometimes, though not so often, adsorbs or adheres to the surface of the disc in the stop mode, with the result that the disc cannot be driven smoothly at the time of starting the operation.
The above-described problem can be solved by a conventional lifter mechanism whose main components are as shown in FIGS. 1A and 1B. FIG. 1A is a plan view of the lifter mechanism and FIG. 1B is a side view of the lifter mechanism. A disc 1 is provided on the respective surfaces thereof with a head 2 which is usually supported by an arm 3 in the form of a thin plate spring. In the example shown a rod-like lifter 9 is operated in the directions indicated by arrows so that the head 2 can be lifted from the surface of the disc 1. This type of lifter was originally intended to separate the head from the surface of the disc while the disc is in a stop mode. The mechanism of this type makes it possible to separate the head adsorbed by the surface of the disc and start the disc.
However, remarkable popularization of disc storage units is accompanied by increasing demand for reducing their price and for making their construction as simple as possible to improve reliability in operation. At present, there is used mainly a so-called CSS (contact start stop) system which is free of such a specific structure as the above-described lifter mechanism; that is, the head is in contact with the surface of the disc while it is stopped and when it is to be started the disc is started in this state.
Furthermore, in disc storage units with large capacity as recently used, the flatness of the surface of the disc has been extremely improved in order to decrease the amount of lifting of the head to thereby increase its recording density. This has disadvantageously increased the tendency for a sliding surface of the head to be adsorbed on the surface of the disc more readily than is observed conventionally due to influences of humidity and the like. In the case of the CSS system, it is often the case that the surfaces of the disc are protected with a lubricant or the like in order to prevent damage which would be caused by the sliding of the head particularly while it is being started or stopped, with the result that the head tends to be adsorbed on the surface of the disc.
In the case where such adsorption or adhesion occurs in the CSS type disc storage units, not only can the disc not be started but also the surface of the disc may be damaged by the head due to considerable distortion of the arm in the form of a thin plate spring which supports the head if the disc is forcibly started by increasing torque of a spindle motor for driving. For this reason, there have been used conventional spindle motors which can be started at extremely low speed and high torque, by at first forcibly separating or peeling off the head from the surface of the disc forcibly before it is accelerated to a high speed.
Other solutions for solving the problem involved in the CSS type hard disc units or floppy disc units are known. For examples a method in which a head is displaced to remove adsorption or adhesion of the head before starting the disc is described in Japanese Patent Application Publication No. 57-60707 and Japanese Patent Application (Laid-Open) No. 63-155479Further, Japanese Patent Application (Laid-Open) No. 62-140286 discloses a method in which a head displacement instruction is issued simultaneously time with the application of rotational drive to the disc, with the instruction being valid only for a predetermined period of time the case where the rotation of the disc is not detected.
In the method in which the above-described low speed, high torque starting spindle motor is used, another problem arises because the spindle motor becomes large in size, which leads to an increase in price, and a longer time is required for putting the disc in a state of constant speed, with result that start-up of the unit is delayed after an electric source is switched on. In addition, the above-described method is effective when the degree of adsorption or adhesion is relatively low but there still remains a problem that the surface of the disc tends to be damaged due to distortion of the head arm as described above when adsorption or the like becomes strong enough.
According to the method in which the head is displaced to remove adsorption or adhesion before starting the disc, there also occurs a problem that the start-up of the unit is certainly delayed by a time required for the displacement of the head, and that a time is prolonged in which the surface of the disc is abraded by the head. Since the head is almost always driven under an extremely light load without adsorption and adhesion, the head tends to run away from the radially outermost side or innermost side of the surface of the disc. Since the head is displaced to remove adsorption or adhesion before starting the disc, there occurs a problem that the start-up of the unit is certainly delayed by a time required for the displacement of the head.
Furthermore, in the method in which a head displacement instruction is issued simultaneously with the starting of the disc and the instruction is made valid only for a predetermined period if where the rotation of the disc is not detected, the above-described problems could be solved in theory. In actuality, though, the timing at which the displacement of the head is started and the time at which displacement is performed are difficult to set up and control. If these factors are not set up appropriately, various problems tend to occur particularly when strong adsorption and the like occur, so that the starting of the disc does not proceed smoothly, time required for the starting is prolonged considerably, and run away of the head occurs. It is also a problem in that it becomes impossible to lift the head when there is a failure to preclude absorption/sticking, because the displacement of the head is limited only for a predetermined period of time.